This invention relates to electrodeless fluorescent light sources excited by high frequency power. More particularly, this invention relates to electrodeless fluorescent light sources having a planar structure and having for excitation an induction coil which produces minimal far field electromagnetic radiation levels other than visible light.
Conventional high brightness fluorescent lamps provide long life and efficient operation but require large, heavy and expensive ballasting circuits for operation at line frequencies. The low pressure glow discharge in mercury vapor that provides the phosphor excitation in fluorescent lamps is usually powered by a current at the power line frequency between two internal emissive electrodes. Current control is required because of the negative impedance characteristic of the discharge, and this is obtained by means of the series inductive impedance of an iron core ballast. In addition, as one attempts to make small fluorescent lamps, power losses connected with the electrodes become an increasingly large fraction of the applied power. Electrodeless excitation of the glow discharge by radio frequency fields has the potential advantage of providing a light weight system by eliminating the usual ballast. Also, without the usual filaments, lamp life would be increased.
Several approaches to electrodeless fluorescent lamps have been taken in the past. In one approach, frequencies in the range of 10 to 500 KHz were used with ferrite structures designed to link the high frequency magnetic field through a closed loop of plasma discharge. In U.S. Pat. No. 3,500,118 issued Mar. 10, 1970 to Anderson and U.S. Pat. No. 3,521,120 issued July 21, 1970 to Anderson, there are disclosed electrodeless fluorescent light sources which utilize a magnetically induced radio frequency electric field to ionize a gaseous radiating medium. Ferrite cores are utilized to couple energy to the discharge. A great variety of geometries are possible. For example, the use of closed loop ferrite core circuits to minimize stray fields that can radiate was disclosed in U.S. Pat. No. 4,005,330 issued Jan. 25, 1977 to Glascock, Jr. et al.
In a second approach, the frequencies are in the 3 to 300 MHz range, and no ferrites are needed. In U.S. Pat. No. 4,010,400 issued Mar. 1, 1977 to Hollister, radio frequency power is coupled to a discharge medium contained in a phosphor coated envelope by an induction coil with a nonmagnetic core connected to a radio frequency source. Radiation by the magnetic dipole field of the excitation coil is a problem.
A third approach to electrodeless fluorescent light sources, utilizing even higher frequencies in the 100 MHz to 300 GHz range, was disclosed by Haugsjaa et al in pending U.S. application Ser. No. 959,823 filed Nov. 13, 1978 and assigned to the assignee of the present invention. High frequency power, typically at 915 MHz, is coupled to an ultraviolet-producing low pressure discharge in a phosphor-coated electrodeless lamp which acts as a termination load within a termination fixture. Electromagnetic radiation is less of a problem at the higher frequencies of operation because shielding can be accomplished with a fine conductive mesh which blocks only a small percentage of the light output. At lower frequencies of operation, such as those disclosed in the Hollister patent, a heavier conductive mesh is required to accomplish effective shielding because of the reduced skin effect at lower frequencies. The heavier mesh is impractical because more of the light output is blocked.
Regardless of the frequency range utilized for exciting the glow discharge of a fluorescent lamp the control of electromagnetic radiation at the operating frequency and its harmonics is of high priority. In the low frequency range, a lamp system utilizing a free running class C oscillator coupled through a coil or ferrite structure to a discharge radiates harmonics randomly dispersed through the 500-1600 KHz broadcast band and gives severe radio interference. In the higher frequency range the effect is similar, but the interference is to other classes of radio and television services. In general, therefore, the operating frequency should be fixed and chosen for electromagnetic compatibility, the power source should be well shielded with its output filtered to remove harmonics, and the coupling system and glow discharge geometry should be chosen to minimize radiation. The power source aspect of this problem was recognized in U.S. Pat. No. 4,048,541 issued Sept. 13, 1977 to Adams et al wherein a power source for an electrodeless fluorescent lamp was designed to eliminate second harmonics.
Applicant's above-identified application Ser. No. 49,773 disclosed an approach to minimizing radio frequency radiation. Radio frequency currents are induced in a gas discharge by a magnetic field configured to have an essentially zero net dipole moment. An induction coil is shaped to provide a field that has low level in the far field. The discharge vessel or lamp is phosphor coated and has the general shape of a cylinder with a central cavity for insertion of the induction coil, thus forming an electrodeless lamp with two concentric walls. The fill material is usually a mixture of mercury and an inert gas. The discharge in the lamp is in the form of loops of plasma current each inductively driven by one of the maxima of the magnetic field configuration.
While the above-described electrodeless fluorescent light source gives generally satisfactory performance, it has certain disadvantages. Under such excitation ultraviolet light at 2537 A is produced by the mercury vapor discharge and becomes reabsorbed and reemitted many times before reaching the phosphor coated glass walls. This so-called self-trapping process limits the efficiency of the fluorescent lamp operation and depends on both the mean distance to the phosphor coated wall and the mercury pressure, the latter being controlled by the lamp temperature and power loading. The coaxial wall geometry described allows an optimization of the mean distance to the wall, but presents other difficulties resulting from the nonequivalence of the inner and outer wall surfaces. For example, the inner surface is not as well cooled by the ambient, and its phosphor coating runs at a higher temperature than that on the outer wall. Moreover, light emitted by the inner phosphor layer is more apt to be lost during the multiple scattering processes required for it to find its way out of the lamp.